In practice, drilling equipments of the type generally mentioned above are used for percussive drilling or top-hammer drilling, i.e., drilling in which impact motions as well as rotary motions are transferred to a drill bit. The equipments can be assembled in most varying ways in the use of different components that are coupled together via threaded joints of different types. However, usually, the male parts consist of rods and the female parts of coupling sleeves, drill bits or reduction sleeves. In the threaded joints between the different components, frequently trapezoidal threads are preferred instead of so-called rope threads by virtue of the first-mentioned ones can be detached from each other in a considerably simpler way than the last-mentioned ones. However, a problem with trapezoidal threads is that stress concentrations arise adjacent to the concave radius transitions between the two flanks of the thread ridge and the intermediate groove bottom, which results in material fatigue and thereby reduced service life of the component in question. Such stress concentrations, which are accentuated the “sharper” angled the radius transitions are, become particularly troublesome in the female parts, in particular in the first thread turn or turns closest to the front or mouth end of the female part, where the bending loads on the female part are largest.
It is true that the problems with stress concentrations in trapezoidal threaded joints have been solved in a meritorious way by U.S. Pat. Nos. 4,040,756 and 6,196,598, viz. by forming the groove bottoms in the threads of the female part as well as of the male part with a so-called full radius profile of the same type that is found in rope threads, at the same time as the tops or crests of the threads maintain the profile shape that is typical of trapezoidal threads and defined by a straight generatrix that at opposite ends transforms into the two flanks via convex arc lines. In such a way, the flanks will transform directly into a cross-sectionally circular arc-shaped groove bottom, without any radius transitions therebetween at all. In other words, the flanks transform into a softly rounded groove bottom in which the risk of emergence of stress concentrations is minimal or non-existing.
However, a disadvantage of the threads known by said patent documents is that the thread profile is uniform along the entire axial extension of the thread between the front and rear ends. This means that the amount of material in above all the fragile female part is reduced not only in the area where bending loads are found, i.e., in the vicinity of the front mouth end of the female part, but also along the remainder of the extension of the thread up to the rear end. This material reduction impairs unnecessarily the strength and service life of the female part.
The same type of unnecessary material reduction is also found in the male part, but in this case, the same is less important than in the female part.
By U.S. Pat. No. 6,293,360, a trapezoidal thread is previously known, the profile of which successively is altered along parts of a turn. However, in this case, the profile alteration is limited to the exit or entrance of the thread, while the rest of the profile is identical along all turns between the entrance and exit.